Oxygen and carbon discovered in exoplanet atmosphere “blow-off”

The well-known extrasolar planet HD 209458b, provisionally
nicknamed Osiris, has surprised astronomers again. Oxygen and carbon
have been found in its atmosphere, evaporating at such an immense rate
that the existence of a new class of extrasolar planets — “the
chthonian planets” or “dead” cores of completely evaporated gas
giants — has been proposed.

Oxygen and carbon have been detected in the
atmosphere of a planet beyond our Solar
System for the first time. Scientists using
the NASA/ESA Hubble Space Telescope have
observed the famous extrasolar planet HD
209458b passing in front of its parent star,
and found oxygen and carbon surrounding the
planet in an extended ellipsoidal envelope –the
shape of a rugby-ball. These atoms are swept up
from the lower atmosphere with the flow of the
escaping atmospheric atomic hydrogen, like dust
in a supersonic whirlwind.

The team led by Alfred Vidal-Madjar (Institut
d’Astrophysique de Paris, CNRS, France) reports
this discovery in a forthcoming issue of
Astrophysical Journal Letters.

The planet, called HD 209458b, may sound
familiar. It is already an extrasolar planet
with an astounding list of firsts: the first
extrasolar planet discovered transiting its
sun, the first with an atmosphere, the first
observed to have an evaporating hydrogen
atmosphere (in 2003 by the same team of
scientists) and now the first to have an
atmosphere containing oxygen and carbon.
Furthermore the “blow-off” effect observed by
the team during their October and November
2003 observations with Hubble had never been
seen before.

In honour of such a distinguished catalogue
this extraordinary extrasolar planet has
provisionally been dubbed “Osiris.” Osiris is
the Egyptian god who lost part of his body —
like HD 209458b ;- after his brother killed
and cut him into pieces to prevent his return
to life.

Oxygen is one of the possible indicators of
life that is often looked for in experiments
searching for extraterrestrial life (such as
those onboard the Viking probes and the
Spirit and Opportunity rovers), but according
to Vidal-Madjar: “Naturally this sounds
exciting – the possibility of life on Osiris
– but it is not a big surprise as oxygen is
also present in the giant planets of our
Solar System, like Jupiter and Saturn.”

What, on the other hand was surprising was to
find the carbon and oxygen atoms surrounding
the planet in an extended envelope. Although
carbon and oxygen have been observed on
Jupiter and Saturn, it is always in combined
form as methane and water deep in the
atmosphere. In HD 209458b the chemicals are
broken down into the basic elements. But on
Jupiter or Saturn, even as elements, they
would still remain invisible low in the
atmosphere. The fact that they are visible in
the upper atmosphere of HD 209458b confirms
that atmospheric “blow off” is occurring.

The scorched Osiris orbits “only” 7 million
kilometres from its yellow Sun-like star and
its surface is heated to about 1,000 degrees
Celsius.

Whereas hydrogen is a very light element – the
lightest in fact – oxygen and carbon are much
heavier in comparison. This has enabled
scientists to conclude that this phenomenon
is more efficient than simple evaporation.
The gas is essentially ripped away at a speed
of more than 35,000 km/hour. “We speculate
that even heavier elements such as iron are
blown off at this stage as well” says team
member Alain Lecavelier des Etangs (Institut
d’Astrophysique de Paris, CNRS, France).

The whole evaporation mechanism is so distinctive
that there is reason to propose the existence of
a new class of extrasolar planets – the
chthonian planets, a reference to the Greek God
Khton, used for Greek deities from the hot infernal
underworld (also used in the French word autochton).
The chthonian planets are thought to be the solid
remnant cores of “evaporated gas giants,”
orbiting even closer to their parent star than
Osiris. The detection of these planets should
soon be within reach of current telescopes both on
the ground and in space.

The discovery of the fierce evaporation process is,
according to the scientists, “highly unusual,” but
may indirectly confirm theories of our own Earth’s
childhood. “This is a unique case in which such a
hydrodynamic escape is directly observed. It has been
speculated that Venus, Earth and Mars may have lost
their entire original atmospheres during the early
part of their lives. Their present atmospheres have
their origins in asteroid and cometary impacts and
outgassing from the planet interiors,” says Vidal-
Madjar.

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Notes for editors

FACTS ABOUT HD 209458b